The invention relates to a nozzle for spraying liquid media, in particular media of high viscosity, particularly for the food industry and the chemical industry. The nozzle is especially suitable for utilizaiton in fluidized bed spray granulators. All sorts of dispersions, emulsions, suspensions or slurries can be gently sprayed with it.
Nozzle constructions are generally adapted to specific fields of application, so that the technological parameters of a nozzle, for example, the spraying angle, shape of spraying cone, droplet size or throughput, can only be changed within narrow limits.
With the known technical methods, liquids which are to be atomized and made into small droplets are forced at high pressures through nozzles. When exiting from the nozzle, due to relieving of the pressure, there are attained droplets of various sizes (unitary nozzle). In other configurations, liquids are conveyed at a lower pressure to the nozzle and by means of compressed air, which is guided parallel or perpendicular to the jet of liquid, then torn into small droplets (binary nozzle).
Nozzles of such design require a high pressure of the liquid for the spraying of viscous liquids. In order to prevent an "afterdrip" from the nozzle when the spraying process is interrupted, there has to be built into the nozzle an additional pneumatic, electrical or mechanical closure system. By a special correlation between the nozzle for the liquid and the nozzle for the air, it is possible to suction the liquid out of the supply vessel (injection nozzle) by means of the vacuum which the exiting compressed air is creating. DE-PS p558542 describes a nozzle intended for the atomization of colorant, whereby an injection tube extends into a nozzle housing. The spraying air is forced through the nozzle housing and thereby, by means of vacuum, suctions in the liquid from the holes arranged laterally in the injection duct. The liquid runs off on the inside of the nozzle housing and is atomized at the nozzle outlet by the air stream. This nozzle has the disadvantage that it is only suitable for the atomization of thin-bodied media, because the therein operating injection effect is too low to suction media of high viscosity. This solution, furthermore, has the disadvantage that it does not perimt a regulation of the quantity of liquid to be atomized, except by changing the quality of air and the air pressure of the atomizing air. At a constant air quantity and air pressure, there are no other means which would allow a variation of the quantity of liquid being atomized. U.S. Pat. No. 4,256,263 describes a unitary nozzle equipped for high product pressures. This nozzle is provided with a torsional device for the liquid to be sprayed which guarantees an even formation of the spraying cone. Liquids of high viscosity cannot be atomized with this nozzle because the built-in torsional element becomes sticky and unusable with substances of high viscosity. Furthermore, all unitary nozzles have the disadvantage of requiring very high pressures at a narrow angle range (approximately 18 to 22 MPa) in order to obtain a good spray mist.
There is no variation of throughput within this narrow range of action.
Nozzles which produce a liquid spray mist in the shape of a hollow cone are not suitable for utilization in fluidized bed spray granulators. It is of particular importance in fluidized bed spray granulators that the liquid to be atomized is sprayed onto the fluidized bed so that it finely and evenly covers the area in order to prevent clogging of the granulate, which would lead to premature interruption of the granulation.
SU-Patent No. 822,914 describes a nozzle, in which the liquid is mixed with the atomization medium (for example, compressed air) in a partially cylindrical and partially cone-shaped cavity in the shape of a sleeve.
The liquid is forced out of an inner duct through lateral holes into the cavity. The spraying medium, after passing through torsional slits which impart a rotating motion to the medium, also enters into the hollow space.
The spraying is attained through the spraying medium and the liquid exiting the nozzle in a scroll shape. The nozzle has the disadvantage that it produces a hollow cone, which is further facilitated by the arrangement of a baffle plate under the scroll-shaped nozzle outlet.
GB-Patent No. 1,131,459 describes a nozzle intended for the processing of thermoplastic materials, the nozzle consisting of a heatable substance nozzle (nozzle for liquid) with a truncated cone-shaped end and a nozzle for gas. The liquid thermoplastic enters from a number of low openings, which are located in the truncated cone-shaped end, into the inner space of the gas nozzle which is supplied with gas via at least one tangentially discharging gas inlet. Herein, the gas which is flowing in a circular path, mixes with the thermoplastic. The spary is produced through discharge of the mixture via an annular slit. Because this relates to a technology entirely different from fluidized bed granulation, it is not possible to convert a nozzle of this design to the latter. As stated in the publication, in order to obtain the required degree of fineness of the spray, the viscosity of the thermoplastic melt has to be extremely low. This requirement is not only dependent upon the relatively quick hardening of this melt during spraying but, furthermore, it has to be pointed out, the mixing of the two media occurs relatively shortly before discharging at the nozzle orifice so that, therefore, the mixing effect in only slight, which is unfavorable for liquids of high viscosity.
Furthermore, GB-OS No. 2,106,422 describes a nozzle on which, on the generated surface of a truncated cone, there are provided spiral guide channels serving to guide the gaseous medium needed for the spraying. The liquid, in this case, motor fuel, is supplied via a centrally arranged hole. The mixing of the media occurs at the "combustion point" outside of the nozzle, whereby the combustion point is the point where the tip of the truncated cone, imagined as a cone, would be. The truncated cone itself is covered by a respective cap which is provided with a central outlet orifice. A nozzle arrangement o this kind is suitable for extremely thin-bodied media, such as, for example, fuel, which makes possible mixing and spraying outside of the nozzle housing. Media of high viscosity cannot be sprayed with this kind of nozzle, because the liquids, due to their high viscosity, would essentially not mix wit the flowing air.
AT-PS 372 describes a nozzle for the spraying of electrostatically charged powder. The design of the nozzle is essentially such that a cylindrical body on the inside of the nozzle is provided with a helical air guide duct which ends in a narrow annular slit located in the immediate vicinity of the nozzle outlet.
At the center of the nozzle, for supplying the powder, there is provided a duct, at the end of which there are located electrodes for the static charging of the powder. A nozzle of this kind, when used for the spraying of media of high viscosity, exhibits similar deficiencies as the above-mentioned nozzles; mainly, the kinetic energy of the jet of air discharing out of a helical duct is insufficient to spray media of high viscosity according to the required conditions.
Therefore, it is not possible to use such a nozzle for this field of application.
According to GB-Patent No. 1,388,468, there has become known a spray injection nozzle, which is especially intended for liquid fuel of low quality and of high viscosity, such as heavy oil. Essentially, the nozzle is of a configuration such that on a covered truncated cone there are provided a multitude of helical air ducts. In the ducts, viewed in the direction of the flow, or in the vicinity thereof, there are assigned holes for supplying the fuel. The fuel is suctioned in by the air flowing through the ducts and is fluidized with the air. These holes, however, are positioned relatively close to the orifice of the nozzle outlet, so that an intensive mixing of the media air with the fuel can occur only outside of the nozzle. The kinetic energy of the flowing air is thus insufficient to render the fuel fluidized to the point that it can be carried along in finest distribution by the air stream. When media of high viscosity, such as have to be processed in the food industry or in the chemical industry, are to be sprayed by such a nozzle, on the one hand, it leads to an extremely poor mixing of the media in question, and on the other hand, the formation of the respective spray cone does not occur. It leads to a coarse drop formation of the media to be sprayed. Furthermore, media of high viscosity are only very poorly suctioned in by the jet of air flowing past it. Even though it is possible to speak of the high viscosity of fuel, it is indeed much lower than that of media being processed in the mentioned industries. Therefore, it is not possible to use this nozzle. Furthermore, according to DE-PS No. 922,039, there is known an adjustable nozzle for atomizing liquid, slurry or powdery substances, whereby there are also provisions for the supplying of the medium to be sprayed as well as for the supply of the air. For this purpose, in the nozzle head there are provided two concentrically arranged components. Each component is provided with a helical duct, one of which respectively serves for the guidance of the air and other for the guidance of the medium to be sprayed, which by means of radial holes is being guided from a central duct to the helical duct. The helical ducts are provided with oppositely directed torsional force. It is particular to this nozzle configuration that the two media to be mixed are impinging upon each other only outside of the nozzle housing, because both media are discharged out of separate orifices of the nozzle outlet, which are arranged concentrically relative to one another. The result is a hollow cone. If media of high viscosity are to be sprayed, then this nozzle is not suitable because the mixing process outside of the nozzle housing of such media only occurs incompletely and would lead to a coarse droplet formation.